Poly(butylene terephthalate) copolyester and a process for preparing it

ABSTRACT

Poly(butylene terephthalate) copolyester advantageous of use as sheath materials in bicomponent fibers are prepared by a process for preparing a high-molecular weight, linear copolyester by condensing 40-85 molar percent of terephthalic acid optionally in the form of a dialkyl ester, half of which component can optionally be replaced by another dicarboxylic acid also optionally in the dialkyl ester form, and 60-15 molar percent of a blend of dimethyl adipate, dimethyl glutarate and dimethyl succinate with an alkanediol of 2-6 carbon atoms in its carbon chain, at an elevated temperature in the presence of a conventional catalyst, in a first condensation stage which is an interesterification or esterification stage and a second condensation stage which is a polycondensation stage, whereby a copolyester melt is produce. The process comprises, before or during the first condensation stage, adding 0.005-0.1 molar percent, based on the total acid component, of a C 6  -aryl or C 7  -alkaryl ester of phosphorous acid or of phosphoric acid to the reaction mixture.

BACKGROUND OF THE INVENTION

The present invention relates to a high-molecular weight, linearcopolyesters and in particular, a poly(butylene terephthalate) (PBT)copolymer and a process for preparing it by the condensation of 40-85molar percent of terephthalic acid or an ester thereof which componentcan be substituted to up to 50% by other dicarboxylic acids, and 60-15molar percent of adipic acid with alkanediols having 2-6 carbon atoms inthe carbon chain, in 2 stages at an elevated temperature in the presenceof conventional catalysts.

Such copolyesters, as well as their manufacture, are known in principleand are disclosed in U.S. Pat. No. 4,328,059 which is incorporated byreference.

At high temperatures required for the polycondensation, especially inthe second condensation stage (polycondensation stage), the presence ofadipic acid results in final products which have a pinkishdiscoloration. This is unacceptable to the processor of these productsTherefore, the addition of esters and salts of phosphoric acids inconjunction with talc has been proposed (DOS No. 2,703,417). At a lowadipic acid content of the copolyesters (<15 molar percent based on thetotal acid component), these measures partially improve the color of thefinished products. At higher adipic acid contents, the undesirablediscolorations cannot be avoided to an adequate extent.

Adipic-acid-containing copolyesters exhibit an additional gravedeficiency. In the melt, they do not show adequate stability againstdegradation by thermal oxidation. Yet, such stability is absolutelyrequired, for example, in hot-melt [fusion] adhesives, since the latterare kept in open melt tanks during their processing for a prolongedperiod of time.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide apoly(butylene terephthalate) copolymer which does not exhibit theabove-described disadvantages, as well as a process for preparing it andmethods of using them, e.g., as sheath materials for bicomponent fibers.

Upon further study of the specification and appended claims, furtherobjects and advantages of this invention will become apparent to thoseskilled in the art.

These objects have been attained by condensing 40-85 molar percent ofterephthalic acid optionally in the form of a dialkyl ester, half ofwhich component can optionally be replaced by another dicarboxylic acidalso optionally in the dialkyl ester form, and 60-15 molar percent of ablended product of dimethyl adipate, dimethyl glutarate, and dimethylsuccinate with at least one alkanediol of 2-6 carbon atoms in its carbonchain, at an elevated temperature in the presence of a conventionalcatalyst, in a first condensation stage which is an interesterificationor esterification stage and a second condensation stage which is apolycondensation stage, whereby a copolyester melt is produced, whichcomprises, before or during the first condensation stage, adding0.005-0.1 molar percent, based on the total acid component, of a C₆₋₁₀-aryl, or C₇₋₂₀ -alkaryl ester of phosphorous acid or of phosphoric acidincluding polyphosphoric acid (PPA) to the reaction mixture.

DETAILED DISCUSSION

Suitable aryl moities for the phosphorous or phosphoric acid estersinclude C₆₋₁₀ -aryl, e.g., phenyl, naphthyl etc. or C₇₋₂₀ -alkarylcontaining 1-2 alkyl groups, e.g, tertbutylphenyl, nonylphenyl,cresylphenyl, etc. Mono-, di- and tri-esters are suitable.

Suitable aryl esters of phosphorous acid or phosphoric acid include, forexample, triphenyl, tri-tertbutylphenyl, trinonylphenyl, tricresylphosphates and similar compounds and polymeric species thereof, as wellas the corresponding phosphites. As mentioned, the object of thisinvention can also be attained if only partially esterified compoundsare employed. Preferably, triphenyl phosphate is utilized. The arylesters are preferably employed in amounts of 0.01-0.06 molar percentbased on the total acid component.

The acid component of the copolyesters contains 40-85 molar percent,preferably 50-75 molar percent, of terephthalic acid and 15-60 molarpercent, preferably 25-50 molar percent of adipic acid.

Up to 50 molar percent of the terephthalic acid can be replaced by otheraromatic, cycloaliphatic and/or aliphatic dicarboxylic acids, such asphthalic acid, isophthalic acid; tetra-(hexa-)hydrophthalic acid,-isophthalic acid, -terephthalic acid; oxalic acid, malonic acid,succinic acid, glutaric acid, sebacic acid, 1,10-decanedicarboxylicacid, and others.

All aforementioned acids can be utilized in the form of their C₁₋ C₄-alkyl ester derivatives.

Suitable diols include alkanediols of 2-6 carbon atoms in the carbonchain. Examples include ethylene glycol, propylene glycol, butyleneglycol etc. Ethylene glycol and butylene glycol are preferred. Thealkanediols can also be used in admixture.

The polyester preparative process is basically conventional and unlessspecified otherwise herein, all details of the process of this inventionare in accordance with these prior art procedures, e.g., as disclosed inSorensen and Campbell-- Preparative Methods of Polymer Chemistry,Interscience

Publishers, Inc., New York [1961]: 111-127; Kunststoff-Handbuch[Plastics Manual] 8 [Polyesters], Carl Hanser publishers, Munich [1973]:697, where disclosures are incorporated by reference herein. Thus, thestarting compounds can be, for example, the dimethyl esters of theacids, and the diols, and an interesterification is first carried out,after the addition of a suitable conventional catalyst. Whendicarboxylic acids per se are employed, the esterification can takeplace at the same time or after the interesterification. This processstage is referred to as the first condensation stage hereinabove.Subsequently, the polycondensation, (i.e., the second condensationstage) is executed under vacuum or in a nitrogen stream with furtherheating.

The polyester prepared by the process of this invention have anintrinsic viscosity of 0.5-1.2 dl/g in orthochlorophenol solvent and 8grams of polymer was mixed in 100 ml solvent.

The copolyesters prepared and used according to this invention exhibit asuperior thermal-oxidation stability while retaining good generalproperties. They do not exhibit any color tinge.

The PBT copolyesters as described above are beneficially employed in themaking of synthetic fibers and/or the encapsulation of other fibers.Such copolyesters can be extruded as a thin sheath on a core fiber ofsome other polymer such as poly(ethylene terephthalate) polyamide,polypropylene, and the like. Of special preference in this disclosure isthe spinning of bicomponent fibers which have a core of PET or PBTcovered with a sheath of the PBT copolyester of the present invention.Bicomponent fibers have been made in accordance with the methodologydisclosed in U.S. Pat. application No. 07/454,217 filed Dec. 21, 1989.

In producing the bicomponent fibers having PET as the core around whichthe PBT copolymer is extruded as a sheath, one should bear in mind thattypical commercial PET melts at about 250° C. unless modified with aningredient which lowers the softening point. The melting point of highlycrystalline PET is about 270° C. The softening/melting point of PBTusually depends on its degree of crystallization and can rang from apressure deflection temperature at about 162° C. to above about 225° C.thus, the core of the fiber will solidify at a higher temperature thanthe grafted linear polymer or a blend which contains the linear polymer.

By having a sheath of the bondable PBT copolymer, the polyester fiberscan be more efficiently used in non-wovens and the like where enhancedbonding of fibers is beneficial or otherwise desirable.

Another advantage of using the PBT copolymer as a component orconstituent in a fiber arrangement is found in its batch dyeability. Byhaving dyeable fibers, it becomes possible in many instances to avoidhaving to add color pigments to the polymers prior to fabricating thepolymer into fibers or other end products.

The following data illustrate certain embodiments of the presentlyclaimed invention, but the invention is not limited to the particularembodiments illustrated.

EXAMPLES

Comparative Experiment 1 of the prior art was prepared in accordancewith the amounts shown in the Table as well as Experiment 2 whichexemplifies the present invention.

In each experiment, the raw materials were charged into a reactor undernitrogen purge and the temperature was set at 220° C. The reactionmixtures were agitated and ester interchange started. The reactionmonomer was polymerized after vacuum let down was completed.

Included in the raw materials is DBE-3, a blended product commerciallyavailable from duPont containing about 89% dimethyl adipate, 10%dimethyl glutanate and about 1% dimethyl succinate. TBT is tetrabutyltitanate. TPPO is triphenyl phosphate. IV is intrinsic viscositymeasured in the solvent orthochlorophenol at 8 gm of polymer in 100 mlof solvent at 25° C. The color test was done in accordance with ASTMMethod E308-85.

                  TABLE                                                           ______________________________________                                                      Comparative                                                                   Experiment 1                                                                           Experiment 2                                           ______________________________________                                        DMT, Kg         8.0        8.0                                                DBE-3 Kg        2.4        2.4                                                Butanediol, Kg  5.8        5.8                                                Hexanediol, g   849        849                                                TBT, g          4.7        9.4                                                TPPO, g         --         2.64                                               IV, dl/g        1.06       0.88                                               MP of polymer °C.                                                                      168        169                                                Color                                                                         L* (brightness) 90.15      92.47                                              A* (red/green)  3.35       0.42                                               B* (blue/yellow)                                                                              11.95      4.05                                               ______________________________________                                    

Resulting polymer from the Comparative Experiment 1 was pink. Resultingpolymer from Experiment 2 had acceptable color. In particular A* and B*were both significantly lower than the Comparative Experiment 1.

Without further elaboration, it is believed that one skilled in the artcan, using the preceding description, utilize the present invention toits fullest extent. The following preferred specific embodiments are,therefore, to be construed as merely illustrative, and not limitative ofthe remainder of the disclosure in any way whatsoever. In the followingexamples, all temperatures are set forth uncorrected in degrees Celsius;unless otherwise indicated, all parts and percentages are by weight.

What is claimed is:
 1. A process for preparing a copolyester bycondensing 40-85 molar percent of terephthalic acid optionally in theform of a dialkyl ester, half of which component can optionally bereplaced by another dicarboxylic acid also optionally in the dialkylester form, and 60-15 molar percent of a blended product of dimethyladipate, dimethyl glutarate and dimethyl succinate with an alkanediol of2-6 carbon atoms in its carbon chain, at an elevated temperature in thepresence of a conventional catalyst, in a first condensations stagewhich is an interesterification or esterification stage and a secondcondensation stage which is a polycondensation stage, whereby acopolyester melt is produced, which comprises, before or during thefirst condensation stage, adding 0.005-0.1 molar percent, based on thetotal acid component, of a C₆₋₁₀ -aryl or C₇₋₂₀ -alkaryl ester ofphosphorous acid or of phosphoric acid including polyphosphoric acid tothe reaction mixture.
 2. A process of claim 1, wherein before or duringthe first condensation stage, 0.01-0.06 molar percent of the aryl oralkaryl ester is added.
 3. A process of claim 1 wherein the aryl oralkaryl ester is triphenyl, tri-tertbutylphenyl, trinonylphenyl ortricresylphosphates or phosphites.
 4. A process of claim 1 whereintriphenyl phosphate is the aryl ester.
 5. A high molecular weightcopolyester prepared by the process of claim
 1. 6. A process forpreparing a poly(butylene terephthalate) (PBT) copolymer by condensing40-85 molar percent of dimethyl terephthalate (DMT); and 60-15 molarpercent of a blended product of dimethyl adipate, dimethyl glutarate anddimethyl succinate; with butanediol and hexanediol, at an elevatedtemperature in the presence of a conventional catalyst, in a firstcondensation stage which is an ester interchange stage and a secondcondensation stage which is a polycondensation stage, whereby the PBTcopolymer is produced, which comprises before or during the esterinterchange stage adding 0.005-0.1 molar percent triphenyl phosphatebased on DMT.